Precious group metal nanoparticles (PGMNPs) have attracted great interest due to their unique catalytic, electronic, and optical properties. For example, platinum-based catalysts (including platinum nanoparticles, PtNPs) are widely used in automobile emission control, chemical industry processes, in the petroleum industry and in low-temperature fuel cells. See U. A. Paulus, T. J. Schmidt, H. A. Gasteiger, R. J. Behm, J. Electroanal. Chem., 134, 495 (2001); J. W. Yoo, D. J. Hathcock, M. A. El-Sayed, J. Catalysis, 214, 1-7 (2003) and P. K. Jain, X. Huaung, M. A. Ei-Sayed, Acc. Chem. Res., 41, 1578-1586 (2008). Syntheses of Pt nanoparticles (PtNPs) with controlled size and shape provide great opportunities for developing high-performance industrial Pt catalysts. See M. Q. Zhao, R. M. Crooks, Adv. Mater., 11, 217-220 (1999); M. Oishi, N. Miyagawa, T. Sakura, Y. Nagasaki, React. Fund. Polym. 67, 662-668 (2007) and K. Peng, X. Wang, X. Wu, S. Lee, Nano Lett., 9, 3704-3709 (2009).
A number of methods have been developed to synthesize PGMNPs, which include spray pyrolysis, vapor deposition, high-temperature reduction-fusion and wet chemistry synthesis. See X. Xue, C. Liu, W. Xing, T, Lu, J. Electrochem., Soc., 153, E79-84, 2006; P. Sivakumar, I. Randa, T. Vincenzo, Electrochim. Acta, 50, 3312-3319, 2005; D. W. Mckee, Nature, 192, 654, 1961; and A. Siani, K. R. Wigal, O. S. Alexeev, and M. D. Amiridis, J. Catalysis, 257, 5-15, 2008. The wet chemistry synthesis method for the preparation of PGMNPs has attracted significant attention due to its technical simplicity and low cost. Also, the wet chemistry synthesis method can provide opportunities to manipulate: (1) precious group metal (PGM) nanoparticle size and size distribution; (2) nanoparticle morphology; and (3) PGM-based alloy composition and structure by simply controlling reaction ingredients and synthesis conditions.
In a typical wet chemistry synthesis, colloidal PGMNPs are synthesized by reduction of a PGM precursor with a reducing agent in a stabilizer-containing solution. In the past, many efforts have been made to synthesize such colloidal PGMNPs for catalyst applications. See M. Adlim, M. A. Bakar, K. Y. Liew, and J. Ismail, J. Molecular Catalysis A, 212, 141-149, 2004; P. R. Rheenen, M. J. Mckelvy, and W. S. Glaunsinger, J. Solid State Chemistry, 67, 151-169, 1987; and O. V. Cherstiouk, P. A. Simonov, E. R. Savinova, Electrochim. Acta, 48, 3851-3860, 2003. However, the existing synthesis methods for the preparation of colloidal PGMNPs do not meet the requirements for certain catalyst applications. First, the reported syntheses in the literature are usually done with very dilute PGM solutions (˜10−4 M). Thus the concentration of the resulting PGM nanoparticles is too low to be practical for most catalyst preparations. Second, the reported synthesis methods commonly employ halogen-containing PGM precursors and sodium-containing inorganic reductants. As a result, undesired Na+ and halide (e.g., Cl−) ions remain on the catalyst surface after synthesis. Such ions can poison and negatively impact the performance of catalysts prepared in this manner and post-synthesis washing processes are thus needed to completely remove these ions. Third, the reported syntheses mostly use hazardous organic solvents and/or toxic organic reductant species. Further, it may be difficult to obtain nanoparticles having the desired size. For example, only syntheses for 1-3 nm PtNPs have been reported in the literature. In addition, some syntheses are reported to require special apparatus and to be run under harsh and/or difficult-to-control conditions.
Therefore, an environmentally-friendly and size-controlled synthesis of halogen- and sodium-free colloidal PGMNPs with high metal content would be useful.